1. Field of the Invention
The present invention relates to the sphere of engine control and more particularly to the estimation of the burnt gas recirculation rate for a gasoline engine provided with an EGR circuit.
2. Description of the Prior Art
Downsizing gasoline engines currently appears to be the preferred option for reducing the consumption of gasoline engines. In fact, this technology allows shifting the working points of the engine to zones of higher efficiency and thus to limit the pumping losses inherent in the operation of an internal-combustion engine. This type of engine then requires the presence of a compressor driven by a turbine arranged in the exhaust line. Such a device is used to improve air filling of the cylinder and to provide a torque equivalent to that of an engine of conventional displacement. It is thus possible to have the same performances while drastically reducing the consumption.
However, using such a technology greatly increases the risk of engine knock appearance. When the engine runs under full load conditions, the thermodynamic conditions in the combustion chamber may be detrimental to the stability of the mixture and generate auto-ignition thereof. This phenomenon can eventually greatly deteriorate the combustion chamber.
To solve this problem, the ignition advance is usually degraded. This option generates an increase in the gas temperature at the end of the combustion cycle and therefore over the entire exhaust line. Thus, in order to compensate for this phenomenon, the mixture is richened at the intake.
Such a method involves two drawbacks: first, it increases the engine consumption; furthermore, it deteriorates the efficiency of the catalyst arranged downstream from the exhaust manifold, which provides optimal conversion of the pollutants resulting from the combustion when the mixture is in stoichiometric proportions.
In this context, burnt gas recirculation (EGR) from the exhaust to the intake is a promising option. Indeed, feeding burnt gas that has not reacted during combustion into a cylinder of the engine allows decreasing the overall combustion temperature and to limit engine knock appearance. The advantages of downsizing in terms of efficiency and consumption are thus preserved. Moreover, introduction of burnt gas also allows reduction of the temperature of the exhaust gas and therefore to limit the impact thereof on the catalyst or the turbine.
However, such a strategy has a great influence on the overall running conditions of the engine. For example, the air mass trapped in the cylinder is smaller in an EGR configuration since burnt gas takes the place of fresh air in the cylinder. To operate under stoichiometric conditions, it is necessary to adjust the fuel loop to the air loop, and thus to have a precise estimation of the composition of the gas in the cylinder. Such a reasoning also applies to the strategies for controlling variable valve actuators (VVT type) or to the ignition advance strategy. Moreover, this estimation is essential for torque transient management, notably at low loads where too high a proportion of burnt gas can extinguish the combustion.
The presence of a gas composition estimator in the intake manifold is therefore necessary to allow proper combustion control (ignition advance control, gas composition control for example), in particular in transient states.